1. Field of the Invention
The present invention relates to a multilayer optical recording medium and a storage device for recording or reproducing information to or from the recording medium.
2. Description of the Related Art
A compact disc (CD) has one recording layer, and information recorded on the recording layer is read from reflected light of a light beam directed from one side of the disc. Further, a digital versatile disc-ROM (DVD-ROM) having an increased recording capacity over the CD includes a standard having two recording layers, and information is read from these two recording layers by optimizing the reflectances and transmittances of the two recording layers. This dual-layer recording method has an advantage such that information can be read from the two recording layers by using a short-wavelength laser without changing a conventional optical system.
To further increase the recording capacity in the future, research on reading of information from a recording medium having multiple recording layers is being conducted. Regarding DVD media, research on a recording medium having three or more recording layers is being conducted. For example, a write-once medium having four recording layers is reported in Optical Data Storage Symposium ODS2001-Lecture No. MC5. The method described in this report is a method for reading information from each recording layer by optimizing the transmittance and reflectance of each recording layer. Thus, multilayering of recording layers is effective in increasing the recording capacity.
However, all the DVD media now in practical use are media having two recording layers for each, and no media having three or more recording layers for each have been put into practical use. In reading information from each recording layer by using a conventional optical head, a sufficient reflected light quantity is required. Further, to read information from a deeper recording layer, a transmitted light quantity must also be increased. As a result, the light quantity reflected on a desired recording layer relative to the light quantity reflected on each recording layer is reduced more with an increase in the number of recording layers, causing a reduction in S/N in reading information. Further, information read from the recording layers other than the desired recording layer is mixed with the reflected light from the desired recording layer, so that information processing for identifying information read from the desired recording layer may become more difficult. Accordingly, while the recording medium having four recording layers is reported in Optical Data Storage Symposium as mentioned above, it is considered that a recording medium having five or more recording layers is difficult to produce.
In the above-mentioned DVD media, information recorded on each recording layer is read by optimizing the transmittance and reflectance of each recording layer. Other information reading methods have been proposed. For example, U.S. Pat. No. 6,009,065 discloses a method of utilizing fluorescent emission at a focal position to identify a desired recording layer from information on the fluorescent emission. This method uses a recording medium having several stacked recording layers formed of a fluorescent chemical substance. This method utilizes a phenomenon such that the fluorescent chemical substance generates scattered light having a wavelength different from the wavelength of an incident laser beam at only the focal position, and information recorded on the desired recording layer is read at the intensity of this scattered light. This fluorescent chemical substance is normally transparent, so that it has an advantage such that a reduction in light quantity can be suppressed even with an increase in the number of recording layers.
In this patent, the focal position must be moved to near the desired recording layer, for example, to select the desired recording layer. However, due to variations in thickness of the recording medium or due to waviness of the recording medium, for example, it is impossible to optically identify whether the focal position is reliably brought to the desired recording layer. Accordingly, it is necessary to perform a fine adjusting operation such that the desired recording layer is identified from address information included in the fluorescent light to bring the focal position to the desired recording layer. It is also necessary to perform an operation for checking whether the data information obtained is originated from only the desired recording layer. It is therefore considered that there is a limit to speed enhancement for information retrieval.
As another conventional recording medium, U.S. Pat. No. 6,005,838 discloses a recording medium having recording layers responding to various combinations obtained by changing the wavelength or polarization direction of a light source for reading information. This patent uses a cholesteric compound capable of transmitting most light and reflecting light having a specific wavelength and polarization direction. This method has an advantage such that by stacking a plurality of recording layers different in reflection wavelength or polarization direction, a desired recording layer can be selected by changing the wavelength or polarization direction, thereby allowing an increase in recording capacity. However, many light sources having different wavelengths are required as compared with a conventional optical system, and a spectral mechanism for separating information according to wavelengths or a mechanism for switching the polarization direction is also required, thus causing an increase in cost and size of a recording device.
As another conventional storage, there is a wavelength multiplexing, multilayer recording type hologram memory using a photochromic material. For example, Japanese Patent Laid-open No. Hei 9-101735 discloses a device for reproducing information by utilizing an optical recording medium having layers of an optical waveguide and a photochromic material, introducing reference light from the optical waveguide to the photochromic material in which information is recorded, and using reconstructed light obtained by modulating the wavefront of the reference light.
The photochromic material is a material for applying a refractive index distribution by absorption of one photon or two photons. However, there has not yet been developed a photochromic material capable of maintaining modification of characteristics required as a recording medium at room temperature and having high resolution and optical reversibility. Even if such an ideal photochromic material is developed as an information recording material, information cannot be recorded or reproduced by using a conventional recording/reproducing device. In recording information, light beams from at least two light sources must be directed accurately to the medium; otherwise interference fringes are moved to cause a reduction in contrast. Because of wobbling due to eccentricity or vibrations of an optical disk medium, it is difficult to adopt such a recording medium using a photochromic material.